This invention relates to methods for measuring visibility from a moving vehicle by image processing techniques.
Most of the information required by a vehicle driver to guide the vehicle is registered visually. The importance of visibility to the driver becomes especially apparent in circumstances in which visibility of the environment to the driver is greatly impaired as, for example, in dense fog or darkness. In a driver's estimate of instantaneous visibility, great errors occur frequently and such errors are aggravated by the absence of reference points such as guideposts at the roadside. But even with such reference points, serious misapprehensions may result because of wide variations in the degree to which the visibility of reference points such as guideposts is impaired because they are faded or soiled. For this reason, the possibility of providing a system for automatic on-board visibility detection in a vehicle is an important consideration. Such a system can be of value not only to assist a human vehicle driver but also for a fully automatic vehicle guidance system.
Various systems for determination of visibility from a vehicle are known in the prior art. For example, German Patent No. DE 37 35 267 describes an arrangement for visibility measurement in which a light emitter, preferably a laser diode, and a light sensor are mounted on a rear-view mirror inside the vehicle. The laser diode produces extremely brief flashes of light aimed though a region of the windshield in the direction of vehicle travel. Depending on the degree to which the windshield is soiled, more or less of the emitted light is diffusely reflected from the windshield. Furthermore, if the vehicle is traveling through a foggy area or approaching a curtain of fog, or if visibility is limited by other matter suspended in the atmosphere, for example smoke, more or less of the light which passes through the windshield is diffusely reflected before it reaches a recognizable object to be detected such as a guidepost.
Light rays reflected back to the sensor by the windshield, the fog or smoke, and the object to be detected, cause the sensor to generate signals representing the level of light intensity picked up from time to time. Moreover, the shorter the visibility, the greater the intensity of the reflected light rays. The arrangement described in this German patent determines the visibility only indirectly, as a function of transmission through the atmosphere, or of the degree to which the windshield is soiled, only those reflected rays which are incident on the sensor being available for analysis.
Another arrangement for determining visibility based on analysis of emitted light rays is disclosed in German Patent No. DE 3,810,840. In that arrangement the physical visibility is determined by using an optical pulsed or continuous radar signal and measuring the range to an object by the transit time method, using parameters of atmospheric transmission, a signal from a brightness sensor, and a searchlight signal. This method is also based chiefly on measuring the proportion of the emitted light which is diffusely reflected by particles present in the atmosphere, and, in darkness, the parameter of brightness enters into the calculation of visibility through the searchlight signal and the brightness sensor. Because the searchlight, especially when stopped down, will not illuminate the roadway space completely, only those objects within the searchlight cone are visible unless an object is intrinsically luminous.